The subject invention pertains generally to RF communications and in particular to RF communications employing scanning transceivers.
As is well known, RF communications is fraught with the problem of establishing a reliable and error-free communications path through the atmosphere between the communicating parties. For example, it is well-known that the integrity of HF signals (2-30 MHz) is particularly susceptible to atmospheric vagaries dependent on environmental conditions such as weather, sun spot activity, time of day, etc., as well as distance. Despite this shortcoming, HF communications is extensively used for maritime and avionics communications because it is not restricted to line of sight communications paths as are higher frequencies. To combat the problem of establishing a reliable communication path, various approaches have been used such as employing a plurality of receivers, each one being dedicated to a particular frequency, so that if a calling party cannot establish communications contact on one frequency, he can attempt to do so on another frequency. As pointed out in a paper entitled "Scan-Lock Control of Digitally Synthesized Receivers" by William Nations and Kenneth Kerwin presented at a Radio Technical Commission for Maritime Services Symposium on Apr. 26-28, 1976, the unattractiveness of such an approach is evident because of the cost of the multiplicity of receivers and the increased exposure to equipment failure, particularly when there are a number of coast stations to be monitored. This paper proposes a solution to the problem which entails the use of a scanning receiver which monitors a plurality of frequency channels and then locks into any channel over which a communications signal is directed to that receiver (using an encoded address identifying the receiver commonly referred to as Selcal for selective calling).
The foregoing approach requires the use of frequencies settled upon in advance by the communicating parties. Consequently, it does not consider the optimization of frequency selection on a dynamic or automatic basis to permit the continual updating of frequency channel assignments to compensate for changing conditions. Nor does it address minimizing the effort required by a human operator to establish a reliable communications channel in various situations which can be of crucial importance. For example, in a warfare scenario, a helicopter pilot who may be performing ground surveillance at an extremely low altitude must focus his attention on the terrain so as to avoid impact with tall objects, such as trees. In such a situation, the pilot cannot afford the luxury of looking up tables or manipulating radio controls in order to establish communications contact with another helicopter or with a ground station when initiating or receiving a call. He may require an RF communications path quickly and reliably but without any distracting exercises which would interfere with aircraft operation. Such a a situation exemplifies the desirability of automatically and dynamically establishing optimum RF communications frequencies so as to afford hands-off operation of the transceiver.
With the foregoing in mind, it is a primary object of the present invention to provide new and improved RF communications which permits the optimum frequency for establishing a communication path to be selected automatically and dynamically in a transceiver without intervention by the operator of the transceiver.
It is a further object of the present invention to provide such RF communications through the use of plurality of frequency channels wherein the frequencies assigned to the various channels in one transceiver can be automatically reprogrammed remotely from another transceiver.
It is still another object of the present invention to provide such RF communications wherein upon initiating a call to another transceiver, a calling transceiver automatically reverts to the same channel last used to establish communications contact with the called transceiver.
The foregoing objects as well as others, and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Invention which follows hereinafter together with the appended drawings.